Falls are the leading cause of fatal and non-fatal injuries in the elderly. Each year an estimated 1/3 of older adults fall with the likelihood of falling increasing substantially with advancing age. Our primary objective is to evaluate the comparative effectiveness of a perturbation-based dynamic stability intervention for fall prevention (using the ActiveStep(r) training system) compared to a traditional physical therapy program in reducing the incidence and severity of falls in a geriatric population by performing a multi-center, randomized clinical trial. In 2005, 15,802 elderly patients (> 65 years of age) died as the result of fall-related injuries. According to the 2006 Behavioral Risk Factor Surveillance System (BRFSS) survey, 5.8 million persons aged 65 or older fell at least once during a 3 month period, which represents nearly 16 percent of the elderly population; furthermore nearly a third of those who fell (1.8 million) sustained some form of injury which resulted in either a doctor's visit or restricted activity for at least 1 day. In 2000, the direct medical costs for fall related injuries were estimated to be $19 billion. The burden of fall related injuries is expected to grow such that by the year 2020, the total cost is estimated to become approximately $54.9 billion. External perturbations such as tripping are a major contributor to falls and trip-related falls account for as many as 20% of all hip fractures. The ActiveStep(r) fall prevention training program was developed with NIH funding and consists of a series of increasingly challenging postural disturbances using a microprocessor controlled servo motor that drives a treadmill belt at defined acceleration, velocity, and distance profiles. Initial biomechanical testing of the ActiveStep(r) training program showed significant improvement in trunk kinematics during a single training session. The ActiveStep(r) training program is currently available and billable under Medicare, however there is a lack of direct clinical data showing reduced falls compared to standard physical therapy interventions. Our Specific Aims are to: 1) conduct a multi-central randomized prospective trial comparing a standard physical therapy gait and balance program with a perturbation-based dynamic stability intervention using the ActiveStep(r) training system. The main outcome measure will be the number of subjects who report a fall during the follow-up period; 2) to assess the longevity of any improvement with the two types of fall prevention programs to gain insight into the potential need for and timing of retraining for fall prevention; and 3) to evaluate underlying patient characteristics which may predict successful prevention of falls help determine the appropriate selection of patients into different types of fall prevention programs.